Sealing composition and method



2,793,967 SEALING COMPOSITION AND METHCID Bruno R. Jeremias, Chicago, 111., assignor to Poor & Company, Chicago, 111., a corporation of @claware No Drawing. Application June 2, 1953, Serial No. 359,217

11 Claims. '(Cl. 148- 62) This invention relates -.to a process for sealing chromic acid treated surfaces, more particularly chromic acid treated zinc surfaces.

It is well known that zinc surfaces tend to turn white or grayish-white with age, a property sometimes referred to as white corrosion," which is apparently due to the formation of oxides and carbonates. .Several processes have been devised for increasing the resistance of zinc surfaces to white corrosion." One of these involvesdipping the article made of zinc or-containin-g a zinc coating in a chromic acid bath. The resultant product, however, discolors and therefore leaves much to be desired.

Several processes have been described in the literature which are said to produce a bright zinc plate. One of these involves the treatment of a zinc ,plated article with a chromic acid bath containing a sulfate radical in a .critical ratio followed by treatment in a second step with a solution of an acid having a pl-lless than 2.68. Another process involves a treatment in one step with a chromic acid bath and in a second step with a caustic alkali solution having a normality between 4.25 and saturation. Still a third process involves a treatment in which glacial acetic acid is employed with chromic acid in a first bath followed by a caustic dip in the secQn'd bath.

The processes which involve a secondary leaching treatment with caustic alkali suffer from he disadvantage that the caustic alkali removes a part of the protecting l P e i y ormed .by the -.ch.r.o. .i a d treatment and hence the resultant articles are less corrosion resistant than might be desired. A secondary treatment using an aqueous solution of alkaline metal silicate as described in U. S. Patent 2,548,420 results in a product of enhanced corrosion resistance but with a slightly bluish tint which is sometimes objectionable to some users.

One of the objects of the present invention is to provide a new and improved method for sealing a chromium.containing film previously applied to a metal article by a chromic acid solution.

Another object .of the invention is to provide a new and improved method for sealing a chromium film of the type described which does not substantially remove the chromium film and results in a product afiording enhanced corrosion protection.

Still another object of the invention is to provide a method of treating metal articles containingachromium film of the type described to seal the chromium film and produce a product having a high specular reflectivity .without substantially sacrificing corrosion protection.

Another object of the inventionis to-provide a methd of employing caustic alkalis in a secondary sealing bath ,of the type described without substantially sacrificing the corr s n pr ection at rded by said fi m- A urther obje of the inventi n i t produ e br gh P ed z u e r c s con a ning a se led-clue mium film of high specular reflectivityfree'fromiridescent and bluish tints. Other objects will appear hereinafter.

tates Patch-t0 ice *Inaccordance-with the invention, it has been found that new and improved results in producing chromium-containing coatings of enhanced corrosion resistance can be obtained by subjecting an article which has been treated with a chrornic acid solution and then rinsed with water to a sealing treatment with an aqueous alkali metal hydroxide solution containing a neutral or alkaline alkali metal salt of an acid dissolved therein, preferably an alkali metal sulfate, an alkali metal chloride or an alkali metal phosphate.

The alkaline solution can be prepared by dissolving an alkali metal hydroxide, for example, sodium hydroxide and/or potassium hydroxide, in water.

The neutral or alkaline salt should be one which is soluble in the alkaline solution, preferably sodium sulfate (Na2SO4). Other '-ioni-zable alkali metal salts which are neutral to alkaline and are soluble in the aqueous alkaline solutioncan be employed, for example, sodium chloride, trisodium phosphate and/ or potassium sulfate. Especially good results have been obtained by employing the neutral and alkaline alkali metal salts of strong inorganic acids, e. g., hydrochloric, sulfuric or phosphoric.

calculated as NaOI-Lper gallon of solution, and a weight The presence of the neutral and/or alkaline alkali metal salts apparently makes it possiblefor the alkali metal hydroxide to leach the alkali-soluble portions of the chromium film Without destroying the capacity of the film to protect the metal surface against corrosion. Thus, under a specific set of conditions, a treatment with a caustic alkali solution containingsodium sulfate resulted in a zinc surfaced metal article protected by a chromium film and having at least twice the resistance to corrosion of the same article prepared under the same conditions except for the omission of the sodium sulfate.

To obtain the optimum results, the quantity of alkali, calculated as NaOH, should correspond to at least two ounces per gallon of the sealing and leaching solution. The quantity of alkali in the solution can be somewhat lower, say 0.5 ounce to one ounce per gallon of sealing solution, provided products having a bluish tint are not o jec ion le.

The sealing andleaching composition can be prepared as a substantially dry mixture of a solid alkaline substance ..and a solid inorganic sulfate, with or without the addition of auxiliary substances such as dextrose. A preferred mixture of this type contains the following ingre i nts:

The quantity of dextrose in this composition can be varied and the dextrose may be omitted entirely.

Goo result are also o in d y ng mpo ition containing;60% sodiurnthydroxide and 40% sodium sulfate, or compositions containing 70% sodium hydroxide and 50% sodium sulfate. Other ionizable alkali metal neutral or alkaline salts which are soluble in aqueous alkali metal hydroxide solutions may be substitute for the sodium sulfate on a chemically equivalent basis. Any appreciable quantity of alkali soluble ionizable alkali metal neutral or alkaline salts added to the alkaline sealing and'leaching solution will serve to improve the corrosion protection afforded by the chromium-containing film.

The maximum quantities of the alkali metal hydroxide and the alkali metal salts employed are determined by the saturation solubilities of the respective materials in water, but in general, good results'have been obtained by employing around two to four ounces of the alkali,

ratio of neutral or alkaline alkali metal salts, calculated as Na2SO4, to alkali metal hydroxide within the range of at least 3:7 to not more than about 1:1. A greater ratio than 1:1 can be used but does not add substantially to the performance.

While the invention is not limited to any theory, it is believed that the alkali metal cations and the anions of the salt in some manner represses the action of the alkali metal hydroxide on some of the components of the film produced by the chromic acid treatment so that valuable corrosion inhibiting components are retained in the film and are not lost in the leaching process. At the same time, components which cause discoloration are removed and it is possible to obtain bright products, for example, on zinc, having a high specular reflectivity and desirable color characteristics.

The invention Will be illustrated but is not limited by the following examples.

Example I Zinc is plated onto sheet steel in a conventional fashion from a conventional bath without adding any brightening agents. A typical conventional zinc plating bath has the following composition:

95l00 grams sodium cyanide 35-55 grams zinc metal (added as the chemically equivalent proportion of zinc oxide) 80-100 grams sodium hydroxide and enough water to make 1. liter of solution 2 grams per liter of zinc dust are stirred into the electrolyte, and the entire mixture is filtered in order to remove traces of heavy metals.

The plated article is then rinsed and dried, or without drying is immersed in a bath having the following composition at room temperature (75 F.):

Chromic acid (CrOs)34 to 46 ounces per gallon of bath Ammonium persulfate (NH4)2S2O33.4 to 4.6 ounces per gallon of bath Licorice powder0.l% to 0.5% by weight of water, and

Sufficient water to make a gallon of solution.

The zinc plated article is immersed in this bath from 1 to 15 seconds and is then thoroughly rinsed and removed from the bath. It will be coated with a yellowbrown iridescent stain, which apparently consists of chromic hydroxide (Cr(OH)3) and various chromium chromates.

After the article has been removed from the first bath, above described, and thoroughly rinsed, it is placed in a second bath having the following composition:

4 to 6 ounces of a mixture of 47.5 by weight sodium hydroxide 47.5% by weight sodium sulfate, and

% by weight dextrose made up to one gallon with water.

The temperature of the second bath is approximately 70 F. to 90 F. The time of immersion is 5 to 30 seconds.

The article is taken from the bath, rinsed and dried. It has high specular reflectivity and excellent corrosion resistance.

Example II The procedure is the same as in Example I except that the chromic acid bath is prepared by dissolving in Water 22.5 ounces per gallon of a composition consisting of 90 parts by weight sodium dichromate, 5 parts by weight sodium sulfate, 2.5 parts by Weight sodium sulfite and 2.5 parts by weight sodium acetate, and also dissolving in said solution 225 cc. per gallon of nitric acid, the remainder being Water.

The zinc surfaced article is immersed in this bath for seconds, rinsed with water, and then given a secondary leaching treatment as described in Example I.

Example III The procedure is the same as Example I except that 47.5% of sodium chloride is substituted for the 47.5% sodium sulfate.

In a similar manner, the secondary treatment can be carried out with other alkaline compositions of the type described having a neutral or alkaline alkali metal salt dissolved therein. Good results are obtained without heating the secondary bath.

The invention is generally applicable to the sealing of films produced on metal articles by treatment with chromic acid solutions. It is especially useful in treating zinc surfaced articles.

In the practice of the invention, it has been noted that the presence of the alkali metal salt substantially reduces the pH of the sealing and leaching solution as compared with the same solution containing no such salt. For example, the sealing and leaching solution of Example I, when used in proportions of 2 ounces, 4 ounces, or 6 ounces of dry composition per gallon of solution, has a pH of approximately 12.3 to 12.4. On the other hand, a solution of 3 ounces of sodium hydroxide per gallon of aqueous solution has a pH of about 13.5. A solution of one ounce of sodium hydroxide per gallon of water has approximately the same pH as a solution of 6 ounces, on a dry basis, of the sealing and leaching composition of Example I per gallon of water but does not remove all of the discoloration from the chromic acid treated surface.

The relative proportions of alkali metal hydroxide and neutral or alkaline salt are preferably such. as to produce a pH between 11 and 13, the optimum being around pH 12.3 to 12.6, in the leaching and sealing solution.

The particular method of applying the chromium films disclosed in Example II is covered broadly and specifically in a copending application, Serial No. 353,677, filed May 7, 1953, and is given herein merely to illustrate other methods of applying chromium films. The present invention is not limited to any particular method of applying the chromium film.

The expression zinc surfaced article is employed herein to cover any article having an exterior surface of zinc, regardless of Whether the article is made entirely of zinc or Whether it merely contains a zinc coating. The expression zinc plated article is employed herein to describe a zinc coated article in which the zinc coating has been applied by an electroplating process.

The invention is hereby claimed as follows:

1. A method of sealing and leaching alkali soluble compounds from chromic acid treated surfaces which comprises applying to such surfaces an aqueous solution consisting essentially of an alkali metal hydroxide and an ionizable neutral to alkaline alkali metal salt dissolved in Water, said salt being a salt of an acid from the group consisting of hydrochloric, sulfuric and phosphoric acids.

2. A method of sealing and leaching alkali soluble compounds from chromic acid treated surfaces which comprises applying to such surfaces an aqueous solution consisting essentially of an alkali metal hydroxide and an ionizable inorganic alkali metal sulfate dissolved in water.

3. A method of sealing and leaching alkali soluble compounds from chromic acid treated surfaces which comprises applying to such surfaces an aqueous solution consisting essentially of an alkali metal hydroxide containing at least 2 ounces per gallon of alkali, calculated as NaOH, and an ionizable inorganic neutral to alkaline alkali metal salt, calculated as Na2SO4, dissolved in water in a weight ratio of said salt to said alkali metal hydroxide within the range of 3:7 to 1:1, said salt being a salt of an acid from the group consisting of hydrochloric, sulfuric and phosphoric acids.

4. A method of sealing and leaching alkali soluble compounds from chromic acid treated zinc surfaces which comprises applying to such surfaces an aqueous solution consisting essentially of an alkali metal hydroxide and an ionizable inorganic alkali metal chloride dissolved in water.

5. A method of sealing and leaching alkali soluble compounds from chromic acid treated zinc surfaces which comprises applying to such surfaces an aqueous solution consisting essentially of an alkali metal hydroxide and an ionizable neutral to alkaline alkali metal salt dissolved in water, said saltbeing a salt of an acid from the group consisting of hydrochloric, sulfuric and phosphoric acids.

6. A method of sealing and leaching alkali soluble compounds from chromic acid treated zinc surfaces which comprises applying to such surfaces an aqueous solution consisting essentially of an alkali metal hydroxide and an ionizable inorganic alkali metal sulfate dissolved in water.

7. A method of sealing and leaching alkali soluble compounds from chromic acid treated Zinc surfaces which comprises applying to such surfaces an aqueous solution consisting essentially of an alkali metal hydroxide containing at least 2 ounces per gallon of alkali, calculated as NaOH, and an ionizable inorganic neutral to alkaline alkali metal salt, calculated as NazSO4, dissolved in water in a Weight ratio of said salt to said alkali metal hydroxide within the range of 3:7 to 1:1, said salt being a salt of an acid from the group consisting of hydrochloric, sulfuric and phosphoric acids.

8. A method of sealing and leaching alkali soluble compounds from chromic acid treated zinc surfaced articles which comprises immersing such articles in an aqueous solution consisting essentially of at least 2 ounces per gallon of sodium hydroxide and a quantity of sodium sulfate in a weight ratio to said sodium hydroxide within the range from about 3:7 to about 1:1.

9. A sealing and leaching composition consisting essentially of an alkali metal hydroxide and an alkali soluble ionizable neutral to alkaline salt dissolved in water, the quantity of alkali metal hydroxide and said salt being sufiicient to produce a pH in said solution of 11 to 13, said salt being a salt of an acid from the group consisting of hydrochloric, sulfuric and phosphoric acids.

10. A sealing and leaching composition consisting essentially of an alkali metal hydroxide and an alkali soluble ionizable sulfate dissolved in water, the quantity of alkali being at least two ounces. calculated as NaOH, per gallon of aqueous solution, the quantity of sulfate. calculated as NazS04, being not substantially greater than the quantity of alkali, and the pH of said solution being not greater than 13.

11. A sealing and leaching composition consisting essentially of sodium hydroxide and sodium sulfate dissolved in water, the quantity of sodium hydroxide being at least 2 ounces per gallon of solution, the weight ratio of sodium sulfate to sodium hydroxide being at least 3:7 but not substantially greater than 1:1, and the pH of said solution being not greater than 13.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD OF SEALING AND LEACHING ALKALI SOLUBLE COMPOUNDS FROM CHROMIC ACID TREATED SURFACES EHICH COMPRISES APPLYING TO SUCH SURFACES AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF AN ALKALI METAL HYDROXIDE AND AN IONIZABLE NEUTRAL TO ALKALINE ALKALI METAL SALT DISSOLVED IN WATER, SAID SALT BEING A SALT OF AN ACID FROM THE GROUP CONSISTING OF HYDROCHLORIC, SULFURIC AND PHOSPHORIC ACIDS. 